This invention relates to a hydrotreating process for converting pitch to conversion process feedstock. It particularly relates to a single stage hydrofining process for converting high sulfur, heavy metals-containing residual oils into suitable catalytic cracking process feedstocks by utilizing a particular stacked-bed catalyst arrangement.
One of the difficult problems facing refiners is the disposal of residual oils. These oils contain varying amounts of pitch, i.e., oils with an atmospheric boiling point above 1000.degree. F., which contain asphaltenes, sulfur and nitrogen compounds and heavy metals (Ni+V) compounds, all of which make them increasingly difficult to process in a conversion process, e.g., a catalytic cracking unit, as the pitch content increases. Asphaltenes deposit on cracking catalyst as coke, which rapidly deactivates the catalyst and requires greater coke-burning capacity. Sulfur and nitrogen compounds are converted to H.sub.2 S, SO.sub.2, SO.sub.3, NH.sub.3 and nitrogen oxides during the cracking process and contaminate the atmosphere. Heavy metals deposit on the cracking catalyst and cause excessive cracking of the feedstocks to gases, thus reducing the yield of more valuable gasoline and distillate fuel oil components. Thus any process which enables refiners to convert a greater quantity of pitch-containing residual oils to gasoline and distillate fuels has great economic benefits.
It is well known that residual oils can be hydrotreated (hydrofined) to reduce the content of deleterious compounds thereby making them more suitable as a catalytic cracking feedstock. However, residual oil hydrotreating processes are very expensive because of rapid deactivation of catalyst and the need for high hydrogen partial pressures, which result in more expensive vessels to accomplish the required reduction of deleterious compounds with existing catalysts. Unless continuous regeneration facilities are provided, such processes require frequent catalyst replacement, which results in process unit downtime and requires larger vessels to process a given quantity of feedstock. If catalyst regeneration facilities are provided, two or more smaller reactor vessels are required so that deactivated catalyst in one reactor may be regenerated while the other reactor(s) continue to operate in the process. Of particular importance is the ability to process residue containing oils in existing hydrotreating units which do not have sufficient hydrogen pressure with existing catalysts to prevent unacceptably rapid catalyst activity loss. Thus improved processes and highly stable catalysts are in great demand.
Several two-stage hydrotreating processes have been proposed to overcome some of the difficulties of hydrotreating pitch-containing residual oils. The five patents discussed below use two catalyst reactor vessels, and are incorporated herein by reference.
U.S. Pat. No. 3,766,058 discloses a two-stage processes for hydrodesulfurizing high-sulfur vacuum residues. In the first stage some of the sulfur is removed and some hydrogenation of feed occurs, preferably over a cobalt-molybdenum catalyst supported on a composite of ZnO and Al.sub.2 O.sub.3. In the second stage the effluent is treated under conditions to provide hydrocracking and desulfurization of asphaltenes and large resin molecules contained in the feed, preferably over molybdenum supported on alumina or silica, wherein the second catalyst has a greater average pore diameter than the first catalyst.
U.S. Pat. No. 4,016,049 discloses a two-stage process for hydrodesulfurizing metal- and sulfur-containing asphaltenic heavy oils with an interstage flashing step and with partial feed oil bypass around the first stage.
U.S. Pat. No. 4,048,060 discloses a two-stage hydrodesulfurization and hydrodemetallization process utilizing a different catalyst in each stage, wherein the second stage catalyst has a larger pore size than the first catalyst and a specific pore size distribution.
U.S. Pat. No. 4,166,026 teaches a two-step process wherein a heavy hydrocarbon oil containing large amounts of asphaltenes and heavy metals is hydrodemetallized and selectively cracked in the first step over a catalyst which contains one or more catalytic metals supported on a carrier composed mainly of magnesium silicate. The effluent from the first step, with or without separation of hydrogen-rich gas, is contacted with hydrogen in the presence of a catalyst containing one or more catalytic metals supported on a carrier preferably alumina or silica-alumina having a particular pore volume and pore size distribution. This two-step method is claimed to be more efficient than a conventional process wherein a residual oil is directly hydrodesulfurized in a one-step treatment.
U.S. Pat. No. 4,392,945 discloses a two-stage hydrorefining process for treating heavy oils containing certain types of organic sulfur compounds by utilizing a specific sequence of catalysts with interstage removal of H.sub.2 S and NH.sub.3. A nickel-containing conventional hydrorefining catalyst is present in the first stage. A cobalt-containing conventional hydrorefining catalyst is present in the second stage. The first stage is preferably operated under conditions to effect at least 50%w desulfurization, while the second stage is preferably operated under conditions to achieve at least about 90%w desulfurization, relative to the initial oil feed sulfur of the first stage. This process is primarily applicable to distillate gas oil feeds boiling below about 650.degree. F. which contain little or no heavy metals.
All of the above referenced patents relate to two stage hydrotreating processes for various heavy hydrocarbon oils utilizing certain advantageous catalysts and/or supports. Some of these patents require interstage removal of H.sub.2 S and NH.sub.3 and others do not. However, none have described a process whereby large quantities of pitch-containing residual oil can be converted into a suitable conversion process, e.g., catalytic cracking feedstock, especially in a single hydrotreating stage. Applicants have found that by using a specific stacked-bed catalyst arrangement containing two different catalytically active compositions, large volumes of high sulfur, metals-containing residual oils can be converted into catalytic cracker feed in a single stage hydrotreating process. This process allows easy conversion of existing single catalytic cracker feed hydrotreater (CFH) reactors to a stacked bed of specified catalysts. The process operates well at hydrogen pressures below 1100 psig, so that no additional high pressure reactors need be constructed. The particular stacked bed combination of catalysts of the invention results in longer runs between replacements or regenerations (increased stability) than would be experienced with either catalyst used alone. Furthermore the stacked bed of the invention has a lower start of run temperature (increased activity) than with either catalyst alone or with other stacked bed combinations.